Telephone system



July 14, 1942.

`H. vH. ABBOTT TELEPHONE SYSTEM Filed March 29, 1941 LOU Ok.

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mm, n QW Patented July 14, 1942 TELEPHONE s-Ys'rEM Henry H. Abbott, Yonkers, N. Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March` 29', 1941, Serial N0. 385,834

33 Claims. v (Cl. 179-27) This invention relates to telephone systems and particularly to automatic switchingI systems for use on subscriber premises, generally known as private branch exchange systems.

The object of the invention is to place at theV tain subscribers but not by others except under permissive conditions, which may be transferred from one called party to another, which may be held while such transfers are being made, and which may be reached automatically, directly or by directive action A feature of the invention is a trunk circuit having a plurality of terminals. VOne terminal extends to the central office where it enters circuits of ordinary and well-known arrangement. A second terminal extends to certain stations within the private branch exchange where direct access to the trunk may be had by such stations. Incidentally, signaling arrangements individual to the trunk appear at each of such stations and in addition also to other stations within the private branch exchange where a given character of service is to be rendered. A third terminal of the said trunk circuit known as the outgoing terminal thereof appears in the automatic switches of the private branch exchange and there is grouped with other similar terminals of other trunks in a hunting group. Those subscribers who have access to such trunks by manipulating their local key in making a call are automatically connected with the first idle trunk at its said outgoing terminal. A fourth terminal of the said trunk circuit known as the incoming terminal thereof appears in the automatic switches of the private branch exchange and may be reached by any of the subscribers only when the trunk has been properly conditioned. This incoming terminal normally tests busy but when the trunk is in the condition set up by an unanswered incoming call or being held on either an incoming or outgoing call will have this busy condition removed so that connection may be made by directive action. A fifth and a sixth terminal of the trunk circuit are provided for transfer service. Any subscriber having connection to the trunk over its second, third or fourth terminal may, by operating his local key, cause the fifth terminal thereof to extend the trunk to an automatic switch which may thereafter connect to another subscriber. Thereafter such other subscriber may, by manipulating his local key,cause the sixth terminal to extend the trunk to, an automatic switch which may thereafter connect to still another subscriber. The said fifth and sixth terminals may be used alternately and indefinitely. When the said local key is manipulated in thev manner above set forth by a subscriber having connection to the said second, third orfourth terminals or to the fth or sixth terminals as set forth, the talking connection to the central ofiice will be severed and a holding condition established so that connection to the trunk at the distant end will not be disturbed. Thereafter, when the newly called subscriber answers, the talking connection to the central oiiice is reestablished and a three-way circuit involving the talker at the central office end, the subscriber at the private branch exchange who has just caused this operation and the newly called subscriber is set up- The subscriber who has caused the transfer or the newly called subscriber may disconnect leaving the connection `under control of the other who may thereafter transfer the call to another; or the subscriber who makes the transfer may disconnect after dialing, whereupon the newly called subscriber will entirely take over control of the trunk circuit.

Other features of the invention are involved in the various types of substations provided. Certain stations. known as attendant stations will be provided with a local key to perform the functions above set forth as well as a key per trunk for gaining access to each trunk over its second terminal. By means of the local key these stations may also gain access to the trunk over their Vthird or outgoing terminals and by means of their dials may gain access to the trunks over their fourth or incoming terminals.

Other stations, which will constitute the majority of the private branch exchange stations will be provided only with a local key and, therefore, cannot gain access to a trunk over its second terminel but in other respects will be like the attendant stations.

Still other stations will not be provided with even a local key and these stations can only gain access to a trunk by dialing and then only when the trunk has been put in condition by having the normal busy condition removed by some previous operation. l

Any one of the three kinds of stations may receive a transferred call over the trunk but the last or keyless station cannot further transfer a call.`

Another feature of the invention is a control karrangement for the connecting facilities whereby when a master station or a key station is operated to automatically select the rst idle trunk such an operation will take place in response to the operation of the local key at such a station. Any attempt by a subscriber at any of the stations to gain access to a trunk by dialing to the automatic trunk hunting position will be balked. By Way of example, access to the trunks in response to the operation of a local key is gained by the automatic movement of a selector-connector to a given level followed by an automatic trunk hunting movement. Should a subscriber knowing the identity of such given level attempt tc reach it by dialing, thinking to circumvent the restricted service provision, he would be prevented from fullling such a design by a control circuit provided for this purpose. Such given leve-l can only be entered by automatic approach thereto. Directive approach automatically bars entrance to such level. Accordingly a feature of the invention is an automatic switch serving a group of outgoing lines which may only be selected automatically.

Another feature of the invention is a transfer circuit which one party may use to repeatedly set up connections to various third parties. In accordance with this feature, the party having gained access to a trunk circuit may by operating his local key to cause the operation of the differential relay in the trunk circuit causes a transferring terminal of the trunk circuit to be extended to a, selector-connector. By then dialing the party may further extend the connection to a wanted party. After this either the calling or the called party may disconnect and the one still holding the connection may again transfer the call providing he has the facility in the shape of the lo-cal key.

Another feature resides in the provision of means to prevent the automatic selection of a central office line on a transferred connection. The line finder selector-connector combination differentiates between calls extended from a calling line and calls extended from a calling trunk. Where the connection has been extended from a calling line, the differential relay in the selectorconnection circuit will effectively respond to the operation of the local key at the substation.

Where, on the other hand, the finder has reline is extended by a line nder to a selectorconnector, and then by operating his local key causes the selector-connector to further extend his line to an idle trunk it is possible by then again operating his local key to place the seized trunk in a holding condition and to have the fifth terminal thereof extended to another selector-connector. Since the action of these automatic swit'ches is swift there is danger that the operation of the local key by the subscriber may extend in time beyond the seizure `of the trunk and thereby cause an unwanted operation. The diiferntial relay in the selector-connector therefore responds to the operation of the local key at the calling substation but-does not become CFI effective until this key is released. Therefore, the differential relay in the trunk Will not be operated immediately upon seizure of the trunk unless there is another operation of the local key by the subscriber after the trunk is seized.

Another feature is a means for dismissing a line finder when it is not wanted. In this system where a subscriber at a key station has control of a trunk he may operate the differential relay in the trunk. This results in placing a hold condition on the trunk toward the central oice and at the same time causes a line finder to extend one of the transfer terminals of the trunk to a selector-connector. In the great majority of cases this operation is what is wanted. However, where the subscriber does not intend to transfer the trunk he may dismiss the line finder by disconnecting after placing the trunk in a holding condition. By way of example a subscriber at a key station may wish to permit a subscriber at a keyless station to have a trunk connection. One way of doing this is to select a trunk by a local trunk key over the second terminal of an idle trunk notifying the subscriber at the keyless station of the number of the trunk he will place in condition. He may then operate his local key to operate the differential relay in the trunk and this will result in firstly placing a holding condition on the trunk toward the central oiiice and secondly causing a line finder to extend the fifth terminal of the trunk to a selector-connector. The transfer facilities are not wanted at this time, however, so the subscriber who has caused this operation may now disconnect and by so doing will dismiss the line finder leaving the trunk still in a holding condition. In this condition the trunk is accessible over its fourth terminal and since the subscriber at the keyless station has been notified that this condition Will be established he may dial the number of the trunk and successfully establish a connection thereto. When the keyless station thus seizes the'trunk the hold condition will be removed and the subscriber put in direct communication with the central office.

The drawings consist of ve sheets of circuit diagram illustrating the present invention.

Fig. l is a schematic circuit diagram showing the complete operation of the system;

Fig. 2 is a circuit diagram showing the station circuit and the station line circuit of the telephones used in this system;

Fig. 3 is a circuit diagram showing the linender circuit;

Fig. 4 is a circuit diagram showing the selector-connector circuit; and

Fig. 5 is a circuit diagram showing the trunk circuit.

The various circuit diagrams are not connected together in a complete system as the use of these various circuits will be apparent from consideration of the schematic of Fig. l.

The telephone system herein disclosed is a system for use on subscriber premises and is in the nature of a private branch exchange. A central oice |00 will be reached over a number of trunk lines here illustrated by the lines HH, |02 and |03. Each line terminates at the subscribers premises in a trunk circuit |94, |05 and |06, respectively. Each of these trunk circuits is similar to that circuit shown in detail in Fig. 5, and each has six terminals therein indicated by the numerals 1-6, inclusive. Terminal of each trunk circuit extends to the central oiice. Terminal 2 of each trunk circuit extends to a certain master station where a connection to the trunk may be made manually. Terminal 3 of each trunk appears in the bank terminals of the selector-connector switch where it may be approached automatically. Terminal 4 of each trunk appears in the bank terminals of the selector-connector switch where it may be approached directively. rIlerminals and 6 appear in the terminals of the line-iinder switch and may be alternately used to extend the trunk to various lines within the private branch exchange.

The switching arrangement within this telephone system consists of a plurality of line finder and selector-connector combinations. The numerals |01 and |08 respectively designate the first line finder and selectoreconnector combination. The numerals |09 and l0 respectively designate a second line finder and selector connector combination and the numerals and ||2 respectively designate the third line nder selector-connector combination. -It will be noted that the bank terminals of the various line iinders are multipled. In the same manner it will be noted that the bank termin-als of the various selector-connectors are similarly multipled.

Various types of lines terminate in the banks of the line-indcr switches so that these line finders will respond to a calling condition set up by these various types of lines and after having found the calling line will extend the connection to the companion selector-connector which may then be set upon the terminals of various types of outgoing lines.

For actual subscriber use there are provided therein, by way of example, telephone stations ||3 to 8, inclusive. Stations ||3 and ||4 are known as master stations and each is provided with a local key such as ||9 and |20, respectively, and a plurality of trunk keys such as those designated |2| to |23 associated with station I3. It will be apparent that if a subscriber at station ||3 operates key |2| such station will be transferred from a connection to the station line circuit |24 to line |25 extending `to terminal 2 of trunk circuit |04 so that the subscriber at this station may obtain direct access to the trunk manually. Likewise, through the operation of keys |22 and |23 connections to lines |26 and |21 leading to terminals 2 of trunk circuits |05 and |06, respectively, may be made.

The subscriber at station ||3 by removing his receiver from its switchhook may operate the station line circuit |24 which will put a calling condition on the associated line-finder terminal and cause one of the line finders to make connection thereto. Assuming that line nder |01 makes connection to this terminal then the subscriber at station ||3 through the operation of his dial may set the selector-connector |08 directively on one of the terminals of that switch which will lead either to one of the terminals 4 of the trunk circuits or one of the lines ||4 to H8 inclusive.

It will also be apparent hereinafter that if the subscriber at station ||3 removes his receiver from the switchhcok and causes the connection of line-nder |01 to his line terminals and then operates his local key ||9 before operating the dial that responsive action in the selector-connector will cause this switch to automatically' hunt over the terminals leading to the No. 3 terminals of the trunk circuits to select the first idle one of these trunks.

Stations ||5 and ||6 are provided with local keys |28 and |29, respectively. These local keys perform exactly the same functions as the local keys associated with stations ||3 and ||l| Therefore it will be noted that the difference betweenfstations ||5 and ||6 and stations H3 and ||4 is that stations ||5 and IIB have no direct manual access to the trunks and therefore must make connection to a trunk through the line-finder selector-connector facilities.

Stations I1 and |8 have no local key and are known as keyless stations. Therefore neither of these stations is able to make connection to the third terminals of the trunk circuits through the automatic selection of the first idle one. Such stations are therefore restricted to making a connection to the terminals 4 of these trunks in the manner to be hereinafter more fully described. It will be noted that under ordinary conditions a bar is established on the terminals of each fourth terminal of these trunks so that a directively established connection to one of these terminals will not give a station access to the trunk except under a particular circumstance. It will hereinafter appear that this bar is removed under two conditions, first when the trunk is in that condition established by an incoming call from the central oii'ice and second when the trunk is being held on a. connection established over one of its other terminals. Thus, if a subscriber, for instance at the station I|1, wishes to make a trunk call, he may Iirst establish a connection by dialing a call to either a master station such as ||3 or a key station such as ||5 and request permission to make such a trunk call. If permission is given the subscriber at station ||3'or 5 will establish a connection to a trunk and after having so established this connection will put it in a holding condition by operation of the local key thereat. After a connection is established from station ||3 or |15 to a trunk 04, by way of example, and the local key ||0 or 28 is depressed an operation of the trunk circuit will put a holding condition on the trunk toward the central oiice and at the same time remove the bar from the fourth terminal thereof so that the station ||1 may then successfully establish a connection through a lineiinder selector-connector combination to the lfourth terminal of the trunk |04.

It will be noted hereinafter that if station ||3 establishes a connection to trunk |04, by way of example, to either the second, third or fourth terminal thereof, or if a station 5 establishes a similar connection to the trunk through its third or fourth terminal and then the local key is operated, such trunk will be put in a holding condition and at the same time fth terminal thereof will become active and a line-finder will be connected thereto. Thereupon the subscriber at station ||3 or l5 may dial the number of any other station in the system, H8, by way of example, and connection will be established thereto. If the station 3' or H5 in making such a connection maintains such a, connection active by holding the receiver off the hook a three-way connection will be established upon the answer of station 8 including station ||3 or ||5, the line t0 the central oifice |0| and station ||8. If station 3 or 5 -then hangs up, its line will be entirely cleared from the now established connection between station 8 over the fifth ter. minal of trunk |04 and the central cnice.

The subscriber at station |8 not having at his disposal a local key, is impotent to further transfer the trunk call. If, however, the call had rst been transferred to a station having a local key, say station H6, by Way of example, the call could have been further transferred. The subscriber at station I I6 finding it desirable to make such a transfer would operate his local key |29 whereupon the sixth terminal of trunk circuit |04 would become active and a line-finder would be connected thereto so that the subscriber at station ||6 would directively operate the associated selector-connector to another subscriber Within this system. If in this case the call is transferred to a station provided with a local key this last station may again transfer the connection by operating the local key whereupon the fifth terminal of the trunk circuit |04 would again become active. In this manner the trunk call may be indefinitely transferred back and fourth between subscribers until nally it is transferred to a substation which has no local key, whereupon this operation would come to an end since such last subscriber is impotent-to operate the transfer circuits of the trunk.

On a call incoming from the central oice, a signal will appear some place on the premises of the subscriber in a signal box |30, |3| or |32. This signal box has in it a pair of signals, one for indicating that the trunk is busy and the other for indicating either an incoming call or the fact that the trunk is in a holding condition. And when either of these two latter indications are being given a connection may be directively established to the fourth terminal of the associated trunk. The signal boxes are not associated with any particular substation necessarily but may be placed at strategic points about the subscribers premises where they may be observed by several subscribers. When the calling condition is being signaled any station which observes this signal may make connection to the fourth terminal of such trunk by the directive operation of a selector-connector. If the signal is observed by a master station such as station ||3 or ||4 a direct manual connection may be made thereto through the operation of the associated trunk key.

It will be noted that each of the station line circuits such as |24 has a terminal both in the line finder bank and selector-connector bank whereby any station within this system may directively set up a connection to any other station therein. The line-finder bank also has in its terminals the transfer terminals of the trunk circuits leading to the terminals and 6 thereof and the selector-connector bank has therein terminals leading to the third and fourth terminals of each of the trunk circuits.

To recapitulate the master stations may at various times be connected to the trunk circuits over terminals 2, 3, 4, 5 an'd 6 thereof and may transfer the trunk to any other station. The busy lamps are within sight of all master stations to facilitate the selection of an idle trunk over its second terminal. The key stations ||5 and H8 may at various times be connected to a trunk over terminals 3, 4, 5 and 6 thereof and may transfer the trunk circuit to other stations. The keyless stations ||l and H8 may at various times be connected to the trunks over terminals 4, 5 and thereof but may not further transfer the connection. Put in another way all stations provided with a local key may automatically gain access to the first idle one of the trunk circuits. No station may directly gain access to the fourth terminals of the trunks unless either the calling or holding signal in the signal box is being displayed. Through the terminals 5 and 5 of the 75- trunk the trunk may be directively extended to any station in the system, but only those stations provided with local keys may further extend the trunk to another station. The fifth terminal of the trunk is always rst to be used and thereafter the sixth and fth terminals are alternately used so long as the trunk is being transferred from station to station.

Fig. 2 shows in detail the station circuit and a station line circuit. The station circuit comprises the handset 200 having a receiver 26| and a transmitter 262 and a set of contacts 263 to 206, inclusive, held in position shown when the handset is on its cradle by means represented by the rod 267. There is also provided an induction coil having windings 298 and 269, a condenser 2|0. ringer 2l| and dial 2|2. A keyless station such as would have the conductors 2|3 and Zl connected directly to the line station circuit conductors 2|5 and 2|G respectively. A key station, such as H5, and a master station, such as H3, would also have a local key 2H' connected so as to ground the tip conductor 2HE. A master station, such as H3, would be provided with a number of keys such as 2|8 which would transfer the conductors 2|3 and 2|4 from conductors 2|5 and ZES, respectively, to conductors 2|9 and 226, respectively, leading to the second terminal of a trunk circuit. It will be understood that as many keys 2i8 as there are trunks may be provided.

The station line circuit comprises a line relay 22| and cut-off relay 222. When the line is idle both of these relays are in their normal unoperated position as shown. When the handset 25! is removed from its cradle then the relay 22| will be energized in the usual manner and will place a ground on the starting conductor 223 leading to the line-nder circuit. At the same time a battery connection through the resistance 224 will be connected to the sleeve conductor 225 so that the line finder may select this calling line by differentiating between the battery connection as a calling condition, the direct ground connection as a busy condition, and the open circuit connection as an idle condition. The conductors 226 and 221 lead to the line terminals of the line finder. Terminals 223, 229 and 232 represent the appearance of the line in the connector banks. It will be noted that when the line is seized over a connector terminal that the cut-off relay is operated. Likewise, when the line finder connects to this line in response to a calling condition thereon, a ground will first be connected over conductor 225 through the contacts of the line relay through the Winding of the cut-off relay and then permanently through the front contact of the outer right-hand armature of relay 222 to the Winding of this cut-off relay. Upon the operation of relay 222, the relay 22| if operated on a calling condition is restored and if not operated, then is thereafter prevented from operating. Upon the restoration of relay 22| on a calling condition ground on the starting lead 223 is removed. It is believed that this short description of the line circuit is sufiicient since this operation is wellknown.

Fig. 3 is a circuit diagram of the line-finder circuit. At the extreme left there is a circuit arrangement including terminals of the linender switch and connections running horizontally to station line circuits and vertically downward to the oflice trunk circuits. This line nder is of that type having two sets of brushes as it serves two groups of calling lines of one hundred in each group. The lower set of terminals 348 to 343. inclusive, serve the tip and ring brushes 381 and 388; the group of terminals 344 to 341, serve the tip and ring brushes 3II and 3I2 and the group of terminals 348 to 35| serve the two alternate sleeve brushes 389 and 3I3. The sleeve brush 389 is associated with the tip and ring brushes 381 and 388 and the sleeve brush 3I3 is associated with the tip and ring brushes 3II and 3I2 When the brushes 381 and 388 come to rest respectively in Contact with terminals 348 and 34I, the brushes 3H and 3I2 will be resting on terminals 344 and 345 and the brushes 389 and 3I3 will be resting on the terminals 348 and 349. In accordance with which one of theA terminals 349 or 349 the calling condition appears on Wll be determined which set of tip and ring brushes will be used.

In the present case let it be assumed that the conductors 221 and 228, 225 and 223 of the station line circuit shown in Fig. 2 are connected to conductors 383, 384, 305 and 386, respectively. This means that the calling condition will be found by brush 389 and that consequently the tip and ring brushes 381 and 388 will be rendered operative. The choice is made by relay 318 which will operate when brushes 381 and 388 are to be used and will fail to operate when brushes 3II and 3I2 are to be used.

When a calling condition appears for instance on conductor 38S, a group relay 3I4 will be operated. This group relay known as the start relay will place a ground on conductor 3| 5 leading to a particular terminal in the bank of terminals generally designated 3I6 to indicate to the line finder in which group the calling line appears, so that the Iirst movement of the line iinder will bea group hunting movement controlled by brush 3I1. Group relay 3I4 also closes a circuit which may be traced from a back contact of relay 3I8 to the winding of relay 3 I 9. The circuit for relay 3I9 may be traced from ground, armature and back Contact of relay 3 I 8, front contact and inner upper armature of group relay 3I4, normal contacts of armature I of relay 328, contacts 2 and I of spring combination 32| which operates on the 11th rotary step of the line nder, winding of relay 3I9 to battery. Relay 3I9 prepares a circuit to ground sleeve lead 322 to the selectorconnector when relay 323 operates, opens the release circuit, prepares a circuit for the upper winding of relays 323 and causes relay 323 to operate over its lower winding. This latter circuit may be traced from ground, armature I and front contact of relay 3I9, the armature and back contact of vertical magnet 324, the armature and back contact of the rotary magnet 325, lower winding of relay 323, armature 2 and back contact of relay 328 to battery. Relay 323 operates the vertical magnet and steps the shaft of the line finder. This circuit may be traced from ground, armature I and front Contact of relay 3I8, front contact and armature of relay 323, armature 4 and back contact of relay 3,26, winding of vertical magnet 324, armature 2 and back contact of relay 328 to battery. Upon its energi- Zation the vertical magnet opens the circuit for the lower winding of relay 323 and this in turn On the rst vertical step the vertical oi -normal springs 321 prepare operating and locking circuits for relays 328 and 3I8. On the upward movement of the shaft the commutator brush 3I1 reaches the segment which is grounded by the operation of the group start relay 3 I 4. When the segment 329 is thus reached relay 323 operates in series with the lower winding of relay 323 and locks in the operated position. This circuit may be traced from ground, front contact and upper armature of relay 3I4, group contact 329, brush 3I1, lower winding of relay 326, armature and back contact of rotary magnet'325, lower winding of relay 323, armature 2 and back contact of relay 328 to battery. A locking circuit for relay 326 may be traced from ground, armature I and front contact of relay 3I8, armature 3r and front contact of relay 326, upper winding of relay 326, winding of vertical magnet 324, armature 2 and back contact of relay 328 to battery.` This holds relay 323l in its operated position to prevent further stepping. Relay 326 is made slow in operating to give a short time between the last vertical step and the rst rotary step toprevent snagging of the brushes due to vibration set up in them during vertical stepping. Relay 323 is slow in operating to prevent it from reoperating the Vertical magnet after its contact closes but before it is fully released. Relay 323 is also slow in releasing to insure its holding when the line nder nds the line.

The release circuit is opened on the operation of relay 326 to preclude the possibility of the release or partial release of the shaft and reoperation of relay 3I9 before the release of relay 326 in which case the rotary magnet would energize without the shaft being centered on any bank level and probably cause jamming of the stationary dog or snagging of the brushes on the bank terminals. The operation of relay 326 also transfers the stepping circuit from the vertical to the rotary magnet through the movement of its armature 4. shaft around until the sleeve brushes reach the terminal which is connected to battery. The circuit for rotary magnet 325 may be traced from ground, armature I and front contact of relay 3I9, front contact and armature of relayl 323, armature 4 and front contact of relay 328, armature 5 and back contact of relay 3I8, the rotary magnet 325 to battery. Here the sameinteraction between relay 323 and the rotary magnet takes place as before described in connection with the action of the vertical magnet so that the rotary magnet automatically rotates the shaft of the line nder until either brush 389 or 3I3 encounters a calling condition heretofore described in connection with the line circuit of Fig. 2.

For the sake of completeness in description it will rst be assumed that brush 3I3 detects a calling condition. Thereupon relay 338 will be operated in series with the upper winding of relay 323 in a circuit which may be traced from battery which is characteristic of the calling condition, brush 3I3, lower winding of relay 338, armature 2 and back contact of relay 3I8, back contact and armature 3 of relay 328, front contact and armature 2 of relay 3I9, upper winding of relay 32,3, front contact and armature I of relay 3I9 to ground. Relay 338 operates and closes a circuit from battery through its upper winding, its front contact and armature 4, armature 3 and front contact of relay 3I9, armature and front contact of relay 323, front contact and armature The rotary magnet thensteps the I of Vrelay '3I9 to ground. The operation of relay 330 closes the tip and ring leads through from the calling line to the selector-connector, opens the release circuit and operates relay 320. The circuit for operating relay 320 may be traced from ground, contacts 3 and 2 of the vertical off-normal contacts 321, front contact and armature 2 of relay 330, the upper Winding of relay 320 to ground. The operation 'of relay 320 closes the sleeve lead 322 through from the rst selector to the line circuit, opens the test circuit to brush 3I3, transfers the start lead from relay 3 I 9 to the lower Winding of relay 320 and removes battery from the relays 323 and 326 and the vertical magnet. This 'allows relay 326 to release. Battery is removed from the lower winding of relay 323 for the purpose of preventing its operation on Athe release of the nder switch. Battery is removed from the vertical magnet 324 to prevent the false operation if Vrelay 326 released before relay 323 rdue to the difference in the releasing time of these two relays.

As soon as relay 330 is operated, and as will appear hereinafter, the relay in theselector-connector is operated which will return a ground over the conductor 322 and this ground will nd its way from conductor 322 over the lfront contact and armature 3 of relay 320, the normal contacts of armature 4 of relay 3I0, armature 3 and front contact of relay 330 to brush 3I3 for the purpose of operating the cut-off relay in the line circuit.

If lit may be assumed that brush 309 instead of brush 3I3 finds the calling condition, then relay 3I0 will operate in series with the upper winding of relay 323. The circuit for relay 3I0 maybe traced from battery Which designates the calling condition on the line brush 309, the upper winding of relay 3I0, back contact and armature 3 of relay 330,'normal contacts of the armature 4 of relay 3I0, `front contact and armature 2 of relay 3I9, upper winding of relay 323, front contact and armature I of 'relay 3I9. The locking circuit for relay 3I0 may be traced from battery, the lower winding Vof relay 3I0, front contact and armature 3 of relay 3I0, contacts 2 and 3 of vertical off-normal springs 321 to ground. The operation f relay 3I0 transfers the tip and ring leads from brushes v3II and 3I2 to brushes 301 and 308, respectively, closes the circuit for operating the relay 330, opens the circuitof Ythe rotary magnet 325 and finally opens the connection from the lower Winding of relay 330 at armature 2 and front contact of relay 3I0. The circuit of the Arotary magnet'325 is opened to prevent the finder taking an additional step should the locking springs of the cut-01T relay in the station line circuit fail to make contact. Through the operation of relay 3l 0, relay 330 becomes energized in a circuit 'from ground, springs 3 and 2 lof the vertical olinormalv springs 321, armature 4 and front contact of relay 3I9, varmature 6 and front contact of relay 3I0, upper winding of relay 330 to battery. Relay 330 then locks in a circuit from its front contact andarmature 4,'armature`3 and front contact of relay 3I9, armature and front Contact of relay 323, front contact and armature I of relay 3I9 to ground. Upon the operation of relay 330, the circuit functions are hereinbef'ore described when battery was found by brush 309 instead of brush 3I3.

In this manner the line finder operates when a calling condition appears on any one of the terminals in its bank and connects the calling line to the selector-connector circuit shown in detail in Fig. 4.

When the subscriber hangs up the receiver before the line is found the relay 3|4 releases allowing the relay 3I9 to release. This closes a circuit for energizing the release magnet, which may be traced from the winding of release magnet 358 through springs 5 and 4 of the vertical off-normal springs 321, armature I and back contact of relay 326, back contact and armature 2 of relay 330, back contact and armature 4 of relay 3I9, springs 2 and 3 of vertical off-normal springs 321 to ground. If this call is abandoned before the brushes have reached the line terminal the nder continues to step during the releasing time of slow release relay 3i9 in which case it may go to the limit of its rotary motion or, if for any other reason, such as an open sleeve circuit, the finder fails to cut through it will continue stepping until it reach-es the 10th contact when the rotary shaft or cam springs 32| will operate on the 11th step and open the circuit to relay 3I9. The rotary shaft springs 32| also short circuit the rotary interrupter springs holding the relay 323 operated which prevents the rotary magnet from deenergizing until relay 3I9 releases. After an interval relay 3I9 releases causing relay 320 to operate and relay 326 to release. Relay 325 completes the closure of the release magnet circuit and the shaft returns to normal. If there is a line Waiting on the 10th contact relay 330 will operate before relay 3I9 releases and relay 3I9 Will then be held operated in series with the puls-e relay of the selector until ground is returned on the sleeve to hold the nder operated. When the subscriber hangs up after Vthe line is found ground is removed from the sleeve lead from the selectorconnector allowing relay 339 to release. Release of relay 330 closes a circuit to energize the release magnet which may be traced from the winding of release magnet 358, springs 5 and 4 of vertical oli-normal springs 321, armature I and back contact of relay 326, back contact and armature 2 of relay 330, back contact and armature 4 of relay 3I9, springs 2 and 3 of vertical oil-normal springs 321 to ground. Relays 328 and 3I0, if operated, are held until the nder returns to normal. The purpose of the lower winding of relay 320 is to hold the relay operated if the finder is returned to normal at a time when the start lead is advanced through this finder circuit and has started a succeeding nder until the iinder started has found the calling line.

When the line finder of Fig. 3 operates its relay 330 to connect a calling line to the selectorconnector, then relay 400 operates by supplying battery and -ground connection to the calling line. The connection may be traced from battery, the upper winding of relay 400, the upper winding of relay 493, back contact and armature 4 of relay 404, conductor 402 and thence through they loop of the calling line to tip conductor 49I, armature I and back -contact of relay 464, lower winding of relay 403, lower winding of relay 400 to ground. Since the calling line at this time maintains a bridge which is effectively a connection between conductors 40! and 402, current will flow through the windings of relays 400 and 493 in such manner that relay 400 will be operated, but relay 403 being differentially wound, will not operate. The operation of relay 400 results in the operation of slow-release relay 405. Relay 405 connects vground through its armature 3 and the calling line.

front contact through the upper winding of re-` lay 405 to sleeve conductor 422. If the calling condition has come from a line circuit there. will be no source of ground in the line circuit. Consequently, after relay 320 operates and while relay 3|9 releases as hereinbefore described, upon the release of relay 3|9 the holding ground for relay 330 is supplied over a circuit which may be traced from ground, armature 3 and front contact of relay 405, the upper winding of relay 406, sleeve conductor 422, sleeve conductor 322, armature 4, and front contact of relay 330 through the winding of relay 330 to battery. If relay 3|0 has been operated then a parallel circuit will be established from sleeve conductor 322 through the front contact and armature 3 of relay 320, front contact and armature 4 of relay 3|0 to brush 309 and thence to the battery connection from the line circuit. If relay 3| 0 has not been operated then a parallel circuit may be traced through the front contact and armature 3 of relay 320, normal contacts of relay 4 of relay 3`|0, armature 3 and front contact of relay 330, brush 3 I3 to the battery condition from Relay 40B will operate in this circuit. The upper Winding of relay 40B is of comparatively low resistance so that after the release of relay 3|9 in the finder circuit and before relay 406 has become operated the ground through its upper winding will effectively hold relay 330 operated and hold the cut-01T relay of the line circuit operated. Relay 406 in Voperat ing locks through its lower winding, its armature I and front contact to the ground supplied over the front contact and armature 3 of slow releasing relay 405. At the same time relay 406 short-circuits its upper Winding by its armature 3 so that the ground supplied by relay 405 will be directly connected to the sleeve conductor 422.

It should be noted that when a trunk connection is made through the line nder, as for instance when a trunk connected to conductors 352, 353, 354 and 306 sets up a calling condition and the b-rushes 3| I, 3|2 and 3|3 are driven respectively into contact with terminals 344, 345 and 349 that a ground connection is extended from the trunk circuit over the sleeve 354 and hence relay 406 in the selector-connector will not be operated because each side of its upper winding will be connected to ground. The ground from the trunk will extend back over conductor 422 and the ground from armature 3 of relay 405 will extend to the other side of the upper winding of this relay. Hence relay 406 discriminates between a connection from a calling line and a connection from a calling trunk, as for instance when trunk |04 is placed in such a condition that a calling condition appears on its fth terminal and the line finder |01 is employed to extend such connection. It will also be noted that the operation of relay 406 will prevent the operation of relay 404 except on calls to levels on which the springs 429 are operated. Relay 404, as will appear hereinafter, is a cutthrough relay and will differentiate between local and trunk calls. On a local call when relay 406 is operated relay 404 is prevented from operating and therefore talking battery will be supplied in one direction from the windings of relay 400 and in the other direction from the windings of relay 401. When a trunk has set up a call to the line nder, then relay 406 is prevented from operating and relay 404 is allowed at the proper time to operate so that now the local supply of talking battery through the windings of relays 400 and 401 will be cut off and the battery from the central oflce will be used.

Let it be assumed that this selector-connector is now to be used for setting up a local connection, as for instance between station ||5 and station ||8.

As soon as the connection between the calling subscriber and the selector-connector has been made va source of dial tone will be connected from conductor 408 through springs and 2 of the vertical oil-normal springs 409, armature I and front contact of relay 400, condenser 4|0 to the tip line conductor 40| and thence to the calling substation. The subscriber thereat recognizing the dial tone will proceed to dial in the usual manner. At each interruption of the line circuit by the dial, relay 400 releases and operates the vertical magnet in series with relay 4| I,

relay 4|| operating on the rst impulse from relay 400. The circuit for the vertical magnet may be traced from ground, armature 2 and back contact of relay 400, armature I and front contact of relay 405, springs 3 and 5 of the vertical ofE-normal springs 409, winding of relay 4| I, winding of vertical magnet 4|2 to battery. Relay 4|| operates so that upon the rst step of the switch and upon the operation of vertical off normal springs 409, the circuit may be traced through springs 3 and 4 of vertical off-normal springs 409, upper armature and front contact of relay 4H, winding of relay 4|| and thence through the vertical magnet. The vertical magnet steps the shaft and brushes to the desired level.

Relays 405 and 4| are not designed to be slow in releasing and do not release on dial interruptions. When the impulses from relay 400 cease relay 4|| releases and transfers the pulse circuit from the vertical magnet to the rotary magnet. On the next series of impulses from the dial relay 400 on releasing operates the rotary magnet in series with relay 4|3, relay 4|3 operating on the first impulse of relay 400. This circuit may be traced from the upper armature of relay 4| I and its back contact, armature and back contact of relay 401, normal contacts of armature 2 of relay 4|4,V winding of relay 4|3, armature 2 and back contact of relay 4|5, winding of rotary magnet 4|6 to battery. Relay 4l3 is designed to be slow in releasing and does not release on dial impulses. The rotary magnet 4|6 steps the shaft and brushes around to the terminals of the called line. Relay 4|3 prepares the busy test circuit and short-circuits the normal contacts of armature 2 of relay 4|4 through which the rotary magnet circuit is taken so that operation of relay 4|4, while the sleeve brush passes over the busy terminals, will not open the rotary magnet circuit.

On the completion of the dial interruptions of the last digit, relay 4|3 being slow in releasing, remains operated a short time after the brushes make contact with the terminals of the called line during which a busy test is made. The test circuit may be traced from the sleeve of the called line, brush 4|1 over the upper armature and front contact of relay 4|3, the armature and back contact of the rotary magnet 4|6, winding of relay 4|4, springs and 2 of the 11th rotary step device 4| 8 through the winding of release magnet 4|9 to battery. If the line is busy, ground will be connected to the brush 4|1 and relay 4I4 will operate. Since the relay 4|4 is of considerably higher resistance than the release magnet 419, relay 414 alone will operate in this circuit.

Assuming for the time being that the line is idle then relay 414 will not be operated. At the release lof relay 413, relay 415 will operate through its lower winding in a circuit which may be traced from battery through the cut-off relay 222 and the sleeve terminal 230 through the sleeve brush of the selector-connector 411, upper armature and back contact of relay 413, lower winding of relay 415, back contact and armature 4 of relay 414, armature 2 and front contact of relay 405 to ground. Relay 415 locks in the circuit from battery, the upper winding of relay 415, front contact and armature 4 of relay 415 to ground on the sleeve lead 422. Relay 415 places a direct ground on the sleeve terminal 411 through its armature 3 and front contact to operate the cut-off relay in the line circuit of the called line. The same ground also places a busy condition on the sleeve terminal in the multiple bank. Relay 415, operated, also opens the rotary magnet circuit so as to prevent further stepping of the brushes if the calling party should again operate the dial. If the called line is busy, relay 414 operates as hereinbefore described and connects the source of busy tone supplied to conductor 420 through armature I and front contact of relay 414 through armature 2 and back contact of relay 401 through condenser 421 through armature 4 and front Contact of relay 4Q@ back contact and armature I of relay 404 to tip conductor 401 whence it extends to the calling subscribers station and notifies the subscriber thereat that the line is busy. The circuit remains in this condition until the receiver at the calling station is replaced on the switchhook,

Assuming the called line to be idle and that it has been seized and its cut-oilC relay operated as hereinabove described, then upon the operation of relay 415 the ringing circuit is connected to the called line from a source of ringing current 432, the upper winding of relay 423, back contact and armature 5 of relay 423, front contact and armature 1 of relay 415, ring brush 424 of the selector-connector and thence to the called line, the return path being traced through the tip brush 425, armature 1 and front contact of relay 415, armature 2 and back contact of relay 423 to ground. Ringing tone is supplied to the calling line by diverting a small portion of the ringing current through condenser 426, front contact and armature 6 of relay 415 to the ring conductor 421 so that the calling subscriber will know that the called subscriber is being rung. The ringing current passing through the upper winding of relay 423 also induces a tone in the lower winding of this relay which finds a path through the upper winding of relay 401 and thence out over the calling line so that the ringing tone supplied through condenser 426 is increased thereby. Relay 423 is designed so as to not operate on ringing current. When the receiver is removed from the switchhook at the called station, the direct current which is superimposed on the ringing current operates relay 423 which thereupon locks through its lower winding, its front contact and armature 3 to the ground on the sleeve conductor 422. Relay 423 in operating cuts 01T the ringing current, closes the talking circuit and short-circuits condenser 426.

Talking battery is now supplied to the calling station through the windings of relay 4011 and to the calling station through the windings of relay 401. The connection is controlled by the calling station. When the receiver at that station is replaced on the switchhook relay 400 releases in turn releasing relay 405. Upon the release of relays 490 and 405 the release magnet 419 is energized and the switch is returned to normal. When the switch finally reaches its normal position the circuit of the release magnet is opened at contacts 6 and 1 of the vertical oitnormal contacts 409.

The selector-connector switch shown in Fig. 4 will also operate in another manner when the subscriber at the calling station operates his local key thus grounding the tip conductor 401. At this time the lower winding of differential relay 403 is shirt-circuited and the current through the upper winding is slightly increased so that the differential relay 403 operates.

Upon its operation relay 403 causes the operation of relay 431 in a circuit from battery, the winding of relay 431, armature and front contact of relay 4037 armature 1 and front contact of relay 4116, front contact and armature 3 of relay 405 to ground. Relay 431 locks through its armature and front contact and supplies ground to the armature of relay 493, so it will not be until the subscriber releases his local key to release relay 403 that the elect of the operation of relay 403 is manifested. This feature is provided so that the selector-connector will not extend the line to a trunk while the local key is still operated and thus cause an unintended operation in the trunk. Now upon the release of relay 403 the ground on its armature is extended through the contacts 428 the back contact and armature of the vertical magnet 412 to the winding of relay 431. In response to this action the selector-connector will automatically step to a particular group and thereafter automatically select the rst idle line in that group. This provision is made so that one of the subscribers having a local key, such as one of the subscribers stations 113 to 116 may automatically select the iirst idle trunk line to the central ofce over its No. 3 terminal. For this purpose and as shown in the present case the No. 3 terminals of the central oice trunks will all be placed on the 9th level of the switch. For the purpose of controlling this automatic action a number of spring combinations operating on certain vertical movements of the shaft of the switch are provided.

Spring combination 428 known as the hunt springs are arranged to be operated when the switch is on its 9th and 10th levels. These springs are provided to change the circuit when the selector-connector reaches the 9th level on its vertical automatic movement under the control of the differential relay 403. Spring combination 429 known as the cut-through springs allow the transmission circuit of the selectorconnector to be changed from local supply of talking battery on connections involving two local stations to the central office battery supply. These springs are arranged to be operated when the switch is in the 8th, 9th and 10th levels. The cut-through operation is carried out on the 8th level since the No'. 4 terminals of the trunks are connected in its 8th level. Spring combination 430 known as the restricted service springs are arranged to be closed on the 9th level only. These springs are provided so that if a subscriber having knowledge that the terminals of the trunks to the central oiiice are situated on the 9th level attempts to circumvent the restricted service provision (characterthrough the front contact and armature 3 of relay 401 to the lower armature of relay 4H. This takes place, of course, as soon as the 9th level has been reached and before relay 4|| has been released. Therefore the ground is connected through the front Contact of the lower armature of relay 4| I through the armature and back contact of the rotary magnet 4|5 to the winding of relay V4|l| causing this relay to operate. The brushes will be automatically stepped around, but due to the ground which is permanently placed n the brush 4|1, the brushes will not stop on an idle trunk and thus the attempt of the subscriber to gain access to a central oiiice line will be frustrated.

The trunk circuit is shown in detail in Fig. 5. The numeral 500 designated the central oice and the tip and ring conductors 50| and 502 constitute the rst terminal of this trunk circuit and correspond to the line designated |0| in the schematic of Fig. 1.

The conductors 503 and 504, respectively, constitute the second terminal. oi this trunk that is schematically represented in Fig. 1 as |25. A pair of conductors 505 and 505 to which lamps, such as 501 andv508, connected to conductor 505 and 500 and |0 connected to conductor 500, represent the signal boxes such as that labeled |30 in Fig. 1. The lamps 500 and 5|0 connected to conductor 506 represent the line lamps and these will ash on an incoming call and remain steadily lighted while the trunk is being held. The lamps 501 and 508 connected to conductor 505 indicate that the trunk is busy.

Conductors 5||, 5|2 and 5|3 constitute the third terminal of the trunk and it may be connected to terminals 435, 434 and 436, respectively, in the ninth level reached by the selector-connector of Fig. 4.

The conductors 5|4, 5|5 and 5|0 constitute the fourth terminal of this trunk and may be connected to terminals such as 435, 434 and 430, respectively, in the eighth level of the selectorconnector of Fig. 4.

The conductors 5|1, 5|8, 5|9 and 520 constitute the fifth terminal of the trunk and may be connected to conductors 352, 353, 354 and 306, respectively, in the line nder of Fig. 3. Conducductor 520 is the starting conductor and is used with both the fifth and sixth terminals of the trunk. Conductors 52|, 522 and 523 constitute the sixth terminal of this trunk and might, for instance, be connected to conductors 355, 350, andI 351, respectively, leading to the line iinder terminals.

The op-eration of this circuit is as follows: The trunk conductor 50| is normally open at the central oce. When the trunk is seized, however, ground is `connected to conductor 50| and this will :complete a circuit through the winding of relay 528, armature 3 and back contact of relay 524, resistance 529 to battery, whereupon relay 528 causes the operation of cutthrough relay 530, which, through its armature 3 places a ground on the sleeve conductor 53|. Relay 538 responds to the operation of relay 530 and through its operation connects battery to conductor 505 to light the busy lamps 501 and 508. When ringing current is applied to the line at the central ofce a path may be traced from conductor .50| through armature 4 and back contact oliT relay 524, winding of ringing relay 525, condenser 523 to ring conductor 502. Relay 525 responds to this current and causes the operation of relay 521 from ground on sleeve conductor 53| to the winding of relay 521. Relay 521 'locks through its front Contact and armature 2, the back Contact and armature of relay 524 to the ground on sleeve conductor 53|. Relay 521 establishes a connection from a source of battery through the interrupter 532, iront contact and armature of relay 521 to conductor 505 whereby the line lamps 509 and 5|0 are caused to flash intermittently, thus notifying any one of the subscribers within the .premises that a call is incoming on the corresponding trunk.

This call may be answered over terminal 2 or 4 of the trunk. Any one of the subscribers such as those at stations H3 and ||4 may manually gain access to the trunk through the operation of their corresponding trunk keys. Any station may dial the number of the trunk and reach it through its terminal 4. In this connection it should be noted that the sleeve conductor 5|0 is normally grounded by a circuit which may be traced from the back Contact and armature 3 of relay 521, thence through the back contact and armature of relay 530, the back contact and armature 3 of relay 54| to conductor 5|6. When an incoming call is received and relay 521 is operated, this ground is removed so that access to the trunk may be gained over its tfourth terminal.

It will further be noted in the ensuing description that when the trunk is put in a held condition that the relay 535 will operate and that this too will disconnect ground from conductor 5|6 and insteadV connect battery thereto through the resistance 550. It may be also incidentally mentioned this time that when the trunk is .being held battery, through armature 9 and front contact of relay 542, will be connected to `conductor 500 to cause the lamps 509 and 5|0 to operate steadily so that whenever lamps 509 and 5|0 are operating either steadily or intermittently, the ground normally connected to the sleeve conductor 5|6 is removed so that a subscriber within the exchange may gain access to the trunk over its fourth terminal by directly dialing to that terminal.

It should be noted that relay 528 is provided so that if the call at the central office should be abandoned the release of this relay will release cut-through relay 530, thus removing `ground from the sleeve 53| and returning the trunk to its normal position.

Let it be assumed that the subscriber at station ||3 answers this incoming call over the second terminal of the trunk by operation of a key such as |2|. The telephone station circuit ||3 will then be bridged across the conductors 503 and 504, which, through the operation of relay 530, are connected to conductors 50| and 502, respectively. This will operate as the usual response of a subscriber to an incoming call and be noted accordingly by the apparatus at the central oice. The windings of the differential relay 533 and the winding of relay 534 are included in this circuit. Relay 534 alone operates, the windings of the differential relay being so poled that they oppose each other in this connection. The operation of relay 534 causes the operation of relay 535 and this relay extends ground from armature 5 and back contact of relay 535 over armature and front contact of relay 535 to the winding of relay 531 which now .becomes operated. Relay 531 causes the operation of relay 524 so that the tip side of the line 50| connected through the winding of relay 528 is now transferred to the back contact of armature 2 of relay 533 but this relay being operated through the operation of relay 530 removes the winding of relay 528 effectively from the connection. Relay 531 causes the operation or relay 54| and this relay now connects ground from the front contact and armature 2 of relay 531 through the front contact andV armature 3 of relay 54| to sleeve conductor 5M;` and also through the front contact and armature 4 of relay 54| to the sleeve conductor 5|3, thereby rendering this trunk inaccessible over its third or fourth terminals. In other words, upon the response orf the subscriber over the second terminal of the trunk, both the third and fourth terminals are rendered artificially busy. Relay 545 is operated over a circuit Ifrom armature 6 of relay 54|, normal contacts of armature of relay 542 to relay 545.

Let it be assumed that the incoming call is actually for a party at, say, station H5. The party at station H3 will therefore operate his local key such as ||9 which, by unbalancing the line, will result in the operation of the differential relay 533. :from ground through its armature and front contact, armature and -back contact of relay 546, armature and vfront Contact of relay 54|, .back contact and armature 5 of relay 536, winding of relay 542 to battery. Relay 542 operates and locks through its armature 5 and front contact to the ground supplied over the back contact and armature 6 of relay 535. It will also be noted that due to the operation of relay 531 and through the front contact and armature 4 or this relay the ground from armature 6 is supplemented by a ground at the back contact of armature of relay 541 when this relay is not operated and by a ground over the front rst contact and armature 6 of relay 546 when this relay is operated.

Relay 542 opens the line between the subscriber at station ||3 and the central oi'ce but through its armature 4 and front contact it connects a holding coil 543 across the central office line. It will be noted that the holding coil 543 connects to the tip of the line 56| ahead of the upper winding of relay 533 but that the front contact of armature 4 of relay 542 is connected to the ring of the line behind the lower winding of relay 533 so that the lower winding only of this differential relay is now included in the holding circuit. Relay 533 therefore remains operated while this holding condition is main tained. At the same time relay 542 connects ground from the armature and back contact of relay 544 through the front contact and armature 3 of relay 542 to the start conductor 525 so that a line finder will hunt for a calling line. The calling condition will appear on terminal 5 of the trunk through the connection of battery through the winding of relay 544, front contact and armature 2 of relay 542, armature and front Contact of relay 545, normal contacts of armature 2 of relay 546 to sleeve conductor 5|9. It will be noted at this time that sleeve conductor 523 is grounded through the front contact and armature 5 of relay 54|. It will also be noted that the transfer relay 545 upon the operation of relay 542 became locked in a locking circuit from ground, armature 3 and front contact of relay 545, armature and front contact of relay 542 to the Winding of relay 545. When a line nder nds the calling terminal 5 of this trunk, relay 544 will be operated and this will remove ground This relay establishes a .circuit from the start conductor 520 and thereupon eX- tend ground through armature 2 and front contact of relay 545 to the sleeve conductor 5|9 so that immediately the -calling terminal is found ground is connected from the trunk into the line nder sleeve in the manner hereinbefore noted. Relay 544 is slow in operating to allow time for the operation of relay 3|0 or 330.

As soon as the line finder has found the calling terminal which, in this instance, connects to conductors 5|1, 5|8 and 5|9, the source of talking battery supply through the windings of relay 460 in the selector-connector will be connected to the tip and ring conductors so that the calling subscriber at the station I3 will now be supplied with talking battery over this path. Relay 546 as well as relay 534 is now included in the circuit and relay 546 will respond with result that relay 541 is operated and due to the ground supplied by armature 6 of relay 536, relay 54|] will follow. Through the operation of relays 535 and 541 the talking battery supply through the resistance coils 548 and 549 and over the front contacts of armatures 1 and 8 of relay 542 will be disconnected from the talking circuit so that the operation of relays 534 and 546 depends now entirely upon the talking battery supply from the selector-connector. Relay 536 now responds in a circuit which may be traced from ground, the armature and front contact of the differential relay 533, armature l and front contact of relay 540, armature 2 and front Contact of relay 54|, front contact and armature 6 of relay 542, winding of relay 535 to battery and ground. Relay 535 immediately locks through its front contact and armature 4 to the ground supplied by the armature of the relay 533.

It should be noted at this point that after the subscriber on the second terminal of this trunk has operated his local key and the differenti-al relay 533 has responded thereto that a holding condition has been put on the trunk toward the central cnice and that the line nder has connected to the fth terminal of the trunk. The subscriber may now disconnect or he may dial another subscriber. Assuming for the time being that he disconnects and thereby dismissesi the line finder, then relay 534 will release followed at proper intervals through the slow-releasing characteristics of relay 535, 531, and 54| in the release of these relays. Relay 545 also releases and this is followed at proper intervals by the release of relays 541 and 546. Relay 535 releases before relay 54|) releases because of the slow releasing characteristics of the relays. Then a holding circuit for relay 546 is momentarily established during the releasing time of relay 531 over a path from battery through the winding of relay 546, back contact and armature 4 of relay 535, front contact and armature 4 of relay 545, front contact and armature 3 of relay 536, armature 2 and front contact of relay 546, armature and front contact of relay front contact and armature I of relay 545, front contact and armature of relay 544 to ground. When relay 531 releases, this holding circuit is opened but another holding circuit for relay 545 remains until the line finder releases, this holding circuit being over conductors 536, 322, 422, top winding of relay 465 and armature 3 of relay 435. When both relay 405 and relay 531 have released, the line finder releases and then relay 545 releases.

The trunk is now being held and this condition is noted rst by the steady illumination of the line lamps 539 and 5H! through the connec- .tion of battery totheconductor555 bythe lfront contact and armaturet of `relay 552. .Secondly,

the normal ground on sleeve conductor 5|@ has been removed and this yconductor now be traced through the armature 3 and back contact of relay 55S, armature |and front contact of relay 535 through-resistance 555 tobatteryvwhich marks this terminal as available. Therefore, the trunk may be seized over its terminala by any one of `the subscribers within the exchahgedialing to this-terminal.

If, after the trunk has beenput in a held condition, and the subscriber within-the exchange has disconnected sothatrelayfti is released, the connection is abandoned the central cnice, then the differential Vrelay-beingheldthrough its .lower -windingwill release. Thisallows relay 555 to releaseand `this inturn opens the holding circuit for relay 525 which in turn allows the cutthrough relay 5,35 Vto release so thatfthe whole trunk circuit-returns to normal.

We have assumed, however, that `the call on the trunk circuit is to be transferred to another station within the exchange, by way of example, station HS. The subscriber at station M3 may now dial the number of wanted station and the connector will respond inthe mannerhereinbefore described. The relays 535, 537, 55.1 and 555 are slow to release andfwill remain operated during the time that the relays 535 and 555 respond to the impulses.

After the second digit of lthe number of the wanted station has been dialed, relay 554 in the selector-connector operates as hereinbeforedescribed. This disconnects battery and ground from the talking circuit. Thereupon relays 555 and 534 release followed by relay 55?. Relay 535 may also release. However, when relay 551 releases, battery and ground are reconnected to the talking circuit through resistances 553 and 559. Then relay 554 reoperates and holds or reoperates relay 535. After relay 551 releases, .relay 540 releases.

The subscriber at station l |.3 may now hang up or he may await the `response of the called subscriber. If he hangs up thenrelays 534, 535, 531 and 54| will all release. Relay 535 will release a short interval after relay 534. Relay 531 will release a short interval after relay 535. A short interval after the release of relay 531 relay 54| releases and this now Vremoves ground from the sleeve terminals 5|3 and 5|5. Sleeve terminal 5|3 is immediately grounded again over armature Il and back co-ntact of `relay 56| to the ground supplied to the sleeve conductor 53|. Sleeve conductor 5|6, however, is left connected through the armature I and front contact of relay 538 through the resistance 555 to battery at this time so rthat this fourth terminal is rendered available within the exchange. This is for the purpose of allowing the trunk to be picked up again by anyone within the exchange if the called party now being rung by the selector-connector reached over the fifth terminal Idoes not answer.

Let it be assumed that in time the subscriber at station H6 responds to the summons of his call bell. It will be noted that since this connection through the line iinder and selector-con nector came from a trunk whereby ground was supplied over the sleeve 5|9 so that the relay 456 had no opportunity to operate and therefore the cut-through relay 455 became operated upon the finding of the called line through the operation of relay 445. The answering called partys telephone will now be included inv a circuit from battery feed coils 548 and 549 through the back contacts of armatures 2 and 3 of relay 535-and the .back contacts of armatures 2 and 3 of relay 557 :and thence through the back contacts of arma- 'tures and 8 of relay 542 to the trunk conduc- 'tors which include the relay 555. Relay- 555 responds to the answer of the called subscriber and this will cause the operation of relay 551. The battery feed for the called subscriber is cut oi by relay 551 but maintained momentarily'by relay 535. Since at this time relay 535is operated, ground through the armature 6 yoi relay 535 is not available yfor the operationof relayfl :and since when relay 551 responds the ground is tdisconnected at the back contact ofarmature of relay 5131, the locking circuit of relay 542 is opened. Relay 55.2 now returns to normal and opens the holding circuit toward the central of- :fice so that the differential relay 553 releases and this in turn causesv the release of relay 535., The lcalled subscriber is'now Vin communication with 4the central office and relay-55.5 is maintained operated through the battery supply from the central oiiice. Upon disconnection of -relay -535 .ground may be traced from armature 6 and back contact thereof through the armature and back contact of relay 535, `armature I and front contact of relay 5M to the winding of relay 55D. Relays 554 and 545 are released upon the release of relay 552, Relay 521| is maintained energized from ground over armature 5 of relay 555.

The sleeve conductor 5| is now grounded at armature 3 and back contact of relay 527. Sleeve conductor 5|3 is grounded through armature 4 and back contact of relay `54| to ground on sleeve conductor 53|. Sleeve conductor 5|5 is grounded from the front contact and armature 2 of relay 56B and sleeve conductorv523 is opened. Thus the third and fourth terminals of this trunkare now rendered unavailable; the line lamps559 and 5| 5 are dark and the busy lamps 55T and 558 are lighted through a battery connection through armature 5 of relay-524 as well as through armature of relay 533.

Let it be assumed now'that the lsubscriber at station H6 wishes to further transfer this connection. Thereupon this subscriber will operate his local key |25 which will resultin the operation of the differential relay 533. Relay 533 will extend ground from its armature and front contact over armature andfront contact of relay 54), armature 2 and back contact of `relay 54|, back contact and armature 5 of relay 535 to the winding of relay 552. Relay 552 now operates and locks in a circuit through its armature 5 and front contact, armature and. back contact of relay 535, front Contact and armature 5 of relay 545 to ground. Relay 552 as before establishes the holding circuit through the resistance 553 and the lower winding of the differential relay 533 so that this differential relay now holds from the central oiiice battery and maintains ground on its front contact. Through the operation of relay 542 ground is placed on the start conductor 520 from the armature and back contact of relay 554. This results in the starting of another line finder. At this time a calling condition is placed on conductor 523 from battery through the winding of relay 554, front contact-and armature 2 of relay 552, armature and back Contact of relay 545, front contact and armaturefof relay 555 to sleeve conductor 523. In the same manner as hereinbefore described the line finder will seek out this calling line and upon finding it will cause the operation of relay 544. Ground will now be extended from armature and front contact of relay 544 through armature 2 and back Contact of relay 545 to the sleeve conductor 523 so that when the line finder makes connection to this trunk a ground will be connected to the sleeve which will result in the failure of relay 406 in the selector-connector to operate. Let it be assumed in this case that the subscriber at station 6, connected now over the fifth terminal of this trunk through the line nder and connector, dials into the selector-connector now connected over the sixth terminal of the trunk. The battery supply will come from the relay 400 in the selector-connector now seized and both relays 534 and 546 will be operated. Dial pulses will operate both of these relays but the slow operating relays 535 and 547 will remain operated so that now all the slow-releasing relays 535, 531, 541, 540 and 54| will be maintained operated. It should be noted that upon the seizure of the sixth terminal by the line finder that relay 536 is operated when relay 54| responded to the operation of relay 534. The difference between this and the case described hereinbefore is that in the present instance the transfer relay 545 is not operated. This is due to the difference in timing between the operation of relays 534 and 546. Where relay 534 is first operated so that relay 54| becomes operated, before relay 542,then the transfer relay 545 becomes operated, but where, as in the present case, relay 546 is first operated and then relay 542 is operated before the relay 54| becomes operated, relay 545 is maintained in its unoperated condition. l

It is believed that with this explanation other various operations of this trunk circuit will be apparent. The transferringof calls from station to station may proceed indefinitely, rst the fifth, then the sixth and then the fifth terminals may be used alternatively and this may go on until some station to which the call has been transferred is unable to operate the differential relay 533.

Stations Hl and H8 are known as keyless stations and they have restricted service, that is, the parties at these stations are now allowed to make a trunk call without permission from some one of the parties at some one of the other stations. In case a party at station H1. wants to make a call, he will rst have to set up a. regular connection to one of the other stations, let us say, by way of example, station H3. There he states his wants and if permission is given he is told to disconnect. Thereupon the party at the master station H3 may manually connect to a trunk circuit or by the use of his local key may automatically connect to a third terminal of the first idle trunk circuit and when he has thus gained access to a central oflice line he will operate his local key again and cause the operation of differential relay 533, thus putting the trunk in a held condition. If the party at station knows which trunk has been selected as by observation of the line lamp 509 or 5|0, the party at Hl may now dial the number of this trunk and gain access thereto over its fourth terminal. If the keyless station H1 is at such a point that observation of the line lamps 509 and 5l0 is impossible, then the party at master station H3, after gaining access to the trunk either over its second or third terminal, may operate his local key so as to operate the differential relay 533 and put the trunk in a held condition and at the .same time cause a line finder to pick up the fifth terminal thereof. The party at station H3 may now dial the number of the station and when this party answers he will be connected to the trunk.

It should be noted that in this or any other of the cases heretofore described, if the party transferring the connection does not h'ang up, then a conference connection will be established including both the party at the central office, the party who has dialed the called party, and the called party. After this either the calling or called party may hang up and the other will maintain the connection. So long as either relay 534 or 54|:` is maintained energized the central office connection will be held and either party may th'erefore further transfer the connection.

Thus, the party to the original connection described as being made over the second terminal of this trunk comprising conductors 503 and 504 transfers the call to another station over the fth terminal thereof comprising conductors 5|1, 5l8 and 5|9. When such called party answers, a conference connection will be established. If the called party in this case hangs up and releases the connection the party over the second terminal of the trunk will maintain the communication with' the central office. This party may again operate his local key and again cause the operation of the differential relay 533 soI that another line finder will pick .up the fth terminal of this trunk.. In this manner a subscriber, say at station H3, may repeatedly extend the trunk to a different called subscriber.

It should be noted that any station within the system having gained access to the trunk circuit of Fig. 5 over any one of the terminals controlling relay 534 may transfer a connection repeatedly as above described or may transfer the connection to a party `over the fth terminal, who may th'en, in turn, transfer the connection to another party over the sixth terminal. Any party having control of relay 534 may transfer the connection to the terminal including relay 546 and vice Versa.

It should be noted furthermore that if a party connected to the second, third, fourth or sixth terminals has operated his local key and, therefore, caused the operation of the differential relay 533 he will seize a selector-connector. At this state it will be possible for such subscriber to again operate his local key to operate the differential relay 403 in the seized selector-connector, but since the associated line finder has responded to the calling condition coming from a trunk, relay 406 will not have been operated and, therefore, the operation of the differential relay 403 will be ineffective so that a call cannot be transferred to another outgoing trunk.

What is claimed is:

1. In a telephone system, a trunk line circuit having a plurality of terminals, a first terminal extending to a distant point, a second terminal manually accessible to a plurality of substation circuits,` a third terminal automatically accessible to a plurality of substation circuits, a fourth terminal directively accessible toa plurality of substation circuits and a fifth and a sixth terminal alternatively extensible to substation circuits.

2. In a telephone system, a trunk line circuit having a plurality of terminals, a i'lrst terminal extending to a distant point, a second terminal manually accessible to a pluralitypf privileged substation circuits, a third terminal automatically accessible to a plurality of substation circuits, a,

fourth terminal directively accessible to a plurality of substation circuits and a fifth and a sixth terminal alternatively extensible to substation circuits, said fourth terminal being normally marked as busy, and means under control of connections made to any other of said terminals for removing said normal busy condition and rendering said fourth terminal accessible.

3. In a telephone system, subscriber stations, automatic switches controlled from said subscriber stations for interconnecting said stations and central oiiice lines, a central office line directively accessible by said switches, said line being normally busy to connections established over said switches, means responsive to an incoming call over said central office line for removing said normal busy condition, and means controlled from said stations over an established connection to said central oflice line for removing said normal busy condition.

4. In a telephone system, subscriber stations, automatic switches controlled from said subscriber stations for interconnecting said stations and central office lines, central office lines each having a plurality of terminals in said switches, means at said stations for causing said switches to automatically connect a calling line to the first idle one of said central office lines over one of said terminals of said central ofiice lines, means controlled from said stations to cause said switches to directively approach another of said terminals of said central office lines, said last terminal being normally busy, means responsive to an incoming cail over one of said central office lines for removing said normal busy condition, means controlled by said stations over a connection established to one of said central office lines over said one of said terminals for removing said normal busy condition whereby connection may be established over said approached terminal.

5.- In a telephone system, subscriber lines, links having incoming and outgoing terminals, switches for interconnecting said lines via said links, central oice lines each having a plurality of terminals accessible by the outgoing terminals of said links, means individual to each said subscriber line for causing an outgoing terminal of a link to automatically connect to one of the terminals ofa central office line, an impulse dial on each said subscriber line, means responsive to dial impulses Yfor causing an outgoing terminal of a link to connect to another terminal of a particular one of said central office lines, said last of said terminals being normally marked busy, means controlled over an established connection to one of said central office lines for removing said normal busy condition whereby a directively established connection to a central cince line may be completed.

6. In a telephone system, subscriber stations, certain of said stations known as key stations being equipped with means to operate a differential relay included in circuits which may be temporarily connected to said stations, others of said stations being not so equipped, link circuits through wliich said stations may be interconnected, a differential relay in each said link circuit, switches for interconnecting said lines via said links under subscriber station control, central ofiice lines each having a plurality of terminals, means in said links responsive to said differential relay for automatically selecting an idle one of said central oice lines, each said central'ofiice line having therein a differential relay responsive to substation control, means for causing said links to directively approach another of said terminals of a central office line, said last terminal being normally busy, and means responsive to said central office line differential relay for removing said normal busy condition whereby a directively established connection to a central oflice line may be completed.

'7. In a telephone system, subscriber stations, certain of said stations known as key stations being equipped with means to operate a differ-ential relay included in circuits which may be temporarily connected to said stations, others of said stations known as keyless stations being not so equipped, link circuits through which said stations may be interconnected, each said link having an incoming and an outgoing terminal and a differential relay controlled over the incoming terminal thereof, switches for interconnecting said lines via said links under substation control, central office lines each having a plurality of terminals accessible to the outgoing terminals of said link circuits and a plurality of terminals accessible to the incoming terminals of said link circuits, means in said lines responsive to the said differential relay therein for automatically selecting an idle one of said central office lines over an outgoing link terminal, means for causing said links to directively approach another of said terminals of a central office line by way of the outgoing terminal of said link, a differential relay in each of said central office lines, means responsive to the said central office line differential relay for connecting one of said links to said central office line over the incoming terminal of the said link circuit, and one of said terminals of the said central office line accessible thereto, means simultaneously responsive to said central office line circuit differential relay for interrupting the talking circuit to said central office, for placing a holding condition on said central office line and for rendering said directively approachable terminal of said central office line circuit accessible.

8. In a telephone system, subscriber stations certain of said stations known as key stations being equipped with means to operate a differential relay included in circuits which may be temporarily connected to said stations, others of said stations known as keyless stations being not so equipped, link circuits through which said stations may be interconnected, each said link having an incoming and an outgoing terminal and a differential relay controlled over the incoming terminal thereof, switches for interconnecting said lines via said links under substation control, central oflice lines each having a plurality of terminals accessible to the outgoing terminals of said link circuits and a plurality of terminals accessible to the incoming terminals of said link circuits, means in said lines responsive to the said differential relay therein for automatically selecting an idle one of said central office lines over an outgoing link terminal, means for causing said links to directively approach another of said terminals of a central office line by way of the outgoing terminal of said link, a differential relay in each of said central office lines, means responsive to the said central office line differential relay for connecting one of said links to said central office line over the incoming terminal of the said link circuit, and one of said terminals of the said central ofiice line accessible thereto, means simultaneously responsive to said central offlce line circuit differential relay for interrupting the talking circuit to said central oiiice, for placing a holding condition on said central oiiice line and for rendering said Vdirectively approachable terminal of said central office line circuit accessible, means for thereafter extending said link circuit by its outgoing terminal to either a key station or a keyless station, means responsive to the answer of said last called station for restoring said interrupted talking circuit and for removing said holding condition, alternatively means responsive to the seizure of said central ofiice line over its said directively approachable terminal for restoring said interrupted talking circuit and for removing said holding condition, and means thereafter responsive to said differential relay for connecting another of said links to said central oflice line over the incoming terminal of the said link and one of said terminals of the said central oiiice line accessible thereto.

9. In a telephone system, subscriber lines leach having a station circuit connected thereto, certain of said station circuits having manually operable means therein for operating a normally inoperative relay in a circuit which may be temporarily connected to said subscriber lines associated therewith, central oiTice lines, cross-bar switches co-mprising coordinate rows of sets of permanently paired contacts for interconnecting said lines, link circuits for completing interconnections of subscriber lines and interconnections of subscriber lines and central oiTice lines, each said link and each said central ohce line circuit including therein a normally inoperative relay responsive to said manually operable means in said certain of said station circuits, means operative on a calling station call when said associated subscriber line has been extended through said cross-bar switch to one of said links and responsive to the operation of said normally inoperative relay therein for automatically operating said cross-bar switches to extend said link to the first idle one of said central oiiice lines, means operative on a calling station call when said associated subscriber line has been extended through said cross-bar switch to one of said links and responsive to direct-ive operation of said calling station circuit for extending said link to a particular one of said central ofce lines, said central oiiice lines presenting a bar to such directively established connections under normal conditions, means under control of other connections to said particular central office line for removing said bar whereby said particular central oiiice line is rendered accessible to said directively established calls, said last means being controlled by said normally inoperative relay in said central office line circuit, means operative on an established connection from one of said subscriber line and link circuit connections established either automatically or directively and responsive to said central orifice line circuit nor-` mally inoperative relay for 'extending said central oiiice line circuit through said cross-bar switches to another of said link circuits, and means for thereafter extending said other of said link circuils through said cross-bar switches to another of said subscriber line stations as a called line.

10. In a telephone system, subscriber lines each having a station circuit connected thereto, certain of said station circuits having manually operable means therein for operating a normally inoperative relay in a circuit which may be temporarily connected to said subscriber lines associated therewith, central ofce lines, cross-bar switches comprising coordinate rows of sets of permanently paired contacts for interconnecting said lines, link circuits for completing interconnections of subscriber lines and interconnection of subscriber lines and central office lines, each said link and each said central oflice line circuit including therein a normally inoperative relay responsive to said manually operable means in said certain of said station circuiis, means operative on a calling station call when said associated subscriber line has been extended through said cross-bar switch to one of said links and responsive to the operation of said normally inoperative relay therein for automatically operating said cross-bar switches to extend said link to the iirst idle one of said central'ofce lines, means operative on a calling station call when said associated subscriber line has been extended through said cross-bar switch to one of said links and responsive to directive operation of said calling station circuit for extending said link to a particular one of said central office lines, said central cnice lines presenting a bar to such directively established connections under normal conditions, means under control of other connections to said particular central oiiice line for removing said bar whereby said particular central ofce line is rendered accessible to said directively established calls, said last means being controlled by said normally inoperative relay in said central office line circuit, means operative on `an established connection from one of said subscriber line and link circuit connections established either automatically or directively and responsive to said central oflice line circuit normally inoperative relay for extending said central ofiice line circuit through said cross-bar switches to another of said link circuits, means for thereafter extending said other of said link circuits through said cross-bar switches to another of said subscriber line stations as a called line, means in said central oiiice line circuit simultaneously responsive to said last-mentioned operation of said central olice line circuit normally inoperative relay to disable the talking circuit over said central oflice line and to place a holding condition thereon, and means responsive to the answer of said called station to remove said holding condition and to reenable said talking circuit over said central oilice line.

11. In a telephone system, subscriber lines each having a station circuit connected thereto, certain of said station circuits having manually operable means therein for operating a normally inoperative relay in a circuit which may be temporarily connected to said subscriber lines associated therewith, central cnice lines, cross-bar switches comprising coordinate rows of sets of permanently paired contacts for interconnecting said lines, link circuits for completing interconnections of subscriber lines and interconnection of subscriber lines and central orlice lines, each said link and each said central oiiice line circuit including therein a normally inoperative relay responsive to said manually operable means in said certain of said station circuits, means oper- Yative on a calling station call when said associated subscriber line has been extended through said cross-bar switch to one of said links and responsive to the operation of said normally inoperative relay therein for automatically operating said cross-bar switches to extend said link to the rst idleone of said central oiilce lines, means 

